Fiber-optic measurement system and methods based on ultra-short cavity length fabry-perot sensors and low resolution spectrum analysis

ABSTRACT

An optical system having an optical sensor with an ultra-short FP cavity, and a low-resolution optical interrogation system coupled to the optical sensor and operational to send light signals and receive light signals to and from the optical sensor is disclosed. The optical system may operate in a wavelength range including the visible and near-infrared range. Optical assemblies and methods of interrogating optical sensors are provided, as are numerous other aspects.

RELATED APPLICATIONS

The present application is a continuation of, and claims priority from,U.S. non-provisional patent application Ser. No. 14/010,675 entitled“FIBER-OPTIC MEASUREMENT SYSTEM AND METHODS BASED ON ULTRA-SHORT CAVITYLENGTH FABRY-PEROT SENSORS AND LOW RESOLUTION SPECTRUM ANALYSIS,”(Attorney Docket No. UNI-MB-007) which was filed Aug. 27, 2013, whichclaims priority from U.S. Provisional Patent Application Ser. No.61/694,902, filed Aug. 30, 2012, entitled “FIBER-OPTIC MEASUREMENTSYSTEM AND METHODS BASED ON ULTRA-SHORT CAVITY LENGTH FABRY-PEROTSENSORS AND LOW RESOLUTION SPECTRUM ANALYSIS” (Attorney Docket No.UNI-MB-007L), each of which is hereby incorporated by reference hereinin its entirety for all purposes.

FIELD

The present invention relates to sensors and measurement systems, morespecifically to optical fiber sensors and methods for interrogationthereof.

BACKGROUND

Fiber-optic sensors (FOS) can provide many advantages over conventionalsensing technologies. Whilst FOSs have been successfully introduced intospecialized markets, such as oil and gas, civil-engineering, energy,military/aerospace, evaluation/testing and similar sectors, penetrationinto a broader range of widespread applications, such as generalindustrial, biomedical, automotive, consumer and similar sectors, mayremain limited. These fields usually require low-complexitysystem-designs and good cost-to-performance-ratios that are difficult toreach with current FOS technologies. However, in general, complexity andcost limitations may not arise from fiber-sensors and fibers, but ratherfrom the complex optoelectronic signal-processing used for sensorintegration. It is common for the costs of signal-integration systems toexceed those of sensor and fiber costs by several orders of magnitude,for example.

Accordingly, to overcome these limitations in the future and to allowfor broader usage of FOS technologies, new approaches to optoelectronicsignal-processing are desired.

SUMMARY

According to first embodiment, an optical system is provided. Theoptical system includes an optical sensor having an ultra-shortFabry-Perot cavity, wherein the ultra-short Fabry-Perot cavity comprisesa cavity length of shorter than 2.5 μm, and an optical interrogationsystem coupled to the optical sensor and operational to send lightsignals to the optical sensor and receive light signals from the opticalsensor within a wavelength range of at least between 400 nm to 1100 nm,which includes visible and near-infrared range, wherein the opticalinterrogation system comprises a spectrally-sensitive detection systemhaving a color detector adapted to receive the light signals andsimultaneously detect light-intensity at more than one wavelength.

In an assembly embodiment, an optical assembly is provided. The opticalassembly includes an optical sensor having an ultra-short Fabry-Perotcavity having an initial cavity length shorter than 2.5 μm, and anoptical interrogation system coupled to the optical sensor andoperational to send light signals to the optical sensor and receivelight signals from the optical sensor within a wavelength range of atleast between 400 nm to 1100 nm, which includes visible andnear-infrared range, the optical interrogation system comprising aspectrally-sensitive detection system having a multi-color detectoradapted to receive the light signals and simultaneously detectlight-intensity at multiple wavelengths.

In a method embodiment, a method of interrogating an optical sensor isprovided. The method includes providing an optical sensor having anultra-short Fabry-Perot cavity, wherein the ultra-short Fabry-Perotcavity comprises a cavity length of shorter than 2.5 μm, andinterrogating the optical sensor by sending light signals to andreceiving light signals from the optical sensor within a wavelengthrange of at least between 400 nm to 1100 nm, which includes visible andnear-infrared range, wherein the interrogating is carried out by anoptical interrogation system comprising a spectrally-sensitive detectionsystem having a color detector adapted to receive the light signals andsimultaneously detect light-intensity at more than one wavelength.

Numerous other aspects are provided in accordance with these and otheraspects of the invention. Other features and aspects of the presentinvention will become more fully apparent from the following detaileddescription, the appended claims, and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a generic ultra-short cavity sensor, according tosome embodiments.

FIG. 2 illustrates the calculated spectra of a 0.75 μm long low-fines FPcavity and a 0.65 μm long low-fines FP cavity, each having a first-orderpeak within the spectrum (m=1) located at a wavelength corresponding toabout 1 μm wherein the change in cavity length of 100 nm shifts the m=1peak position by about 100 nm, according to some embodiments.

FIG. 3 illustrates a calculated spectrum of a 0.25 μm and 0.35 μm longlow-fines FP cavities. The zero-order peak within the spectrum (m=0) ofthe 0.25 μm long cavity is located at a wavelength that corresponds to 1μm. No other peak is present within a 400 nm-1100 nm range in case ofthe 0.25 μm long cavity. The first-order peak (m=1) of 0.35 μm longlow-fines FP cavity is at 467 nm, all other spectral peaks are outsidethe 400 nm-1100 nm range, according to some embodiments.

FIG. 4 illustrates the calculated spectra of the 0.5 μm and 0.6 μm longlow-fines FP cavities. The first (m=1) and the second-order (m=2) peaksare present within a 400 nm-1100 nm wavelength range. A cavity lengthchange from 0.5 μm to 0.6 μm causes the first order peak to shift by 133nm, while the second order peak shifts by about 80 nm, according to someembodiments.

FIG. 5 illustrates the calculated spectra of the 0.75 μm and 0.85 μmlong low-fines FP cavities. The first (m=1), the second (m=2), and thethird (m=3) order spectral peaks are present within a 400 nm-1100 nmwavelength range. The cavity length change from 0.75 μm to 0.85 μmcauses the second-order peak to shift by 80 nm, while the third-orderpeak shifts by about 57 nm, according to some embodiments.

FIG. 6 illustrates the calculated spectra of 1 μm and 1.1 μm longlow-fines FP cavities. The second (m=2), the third (m=3), and the fourth(m=4)-order spectral peaks are present within a 400 nm-1100 nmwavelength range. The cavity length change from 1 μm to 1.1 μm causesthe second-order peak to shift by 80 nm, while the third andfourth-order peaks shift by about 58 nm and 46 nm, according to someembodiments.

FIG. 7 illustrates the calculated spectra of the 1.5 μm and 1.6 μm longlow-fines FP cavities. The third (m=3), the fourth (m=4), the fifth(m=5), and the sixth-order (m=6) spectral peaks are present within a 400nm-1100 nm wavelength range. The seventh-order peak is also presentwithin the same wavelength range in case of the 1.6 μm long cavity. Thecavity length change from 1.5 μm to 1.6 μm causes the third, the fourth,the fifth and the sixth peaks to shift by 57 nm, 44 nm, 37 nm, and 31nm, respectively, according to some embodiments.

FIG. 8 illustrates the calculated spectra of the 2 μm and 2.1 μm longlow-fines FP cavities. The fourth (m=4) to ninth-order (m=9) spectralpeaks are present within a 400 nm-1100 nm wavelength range. The cavitylength change from 2 μm to 2.1 μm causes the spectral peaks to shiftfrom between 44 and 21 nm, according to some embodiments.

FIG. 9 illustrates the basic configuration of a sensing system having abroad-band light source coupled to a multimode fiber, which is furtherconnected to a separator such as a multimode coupler according to someembodiments. The separator is further connected to an ultra-short cavityFabry-Perot sensor, and the spectrum-analysis system, according to someembodiments.

FIG. 10 illustrates a feedback-assisted fusion-splicing system used forthe production of ultra-short FP cavities, according to someembodiments.

FIG. 11 illustrates an all-silica, all-fiber pressure sensor having anultra-short FP cavity, according to some embodiments.

FIG. 12 illustrates a measured FP cavity optical spectrum obtainedbefore and after completion of the splicing process according to someembodiments. The spectrum before splicing was obtained by axiallydepressing a lead-in fiber and an etched sensor-forming fiber togetherby the application of the fusion splicer's axial motors, according tosome embodiments.

FIG. 13A illustrates an all-silica, all-fiber, long active length, butultra-short cavity strain sensor, according to some embodiments.

FIG. 13B illustrates an optical microscopic photograph of an exemplaryproduced sensor, according to some embodiments.

FIG. 14 illustrates the temperature sensor based on the ultra-shortcavity strain sensor, according to some embodiments.

FIG. 15 illustrates a measured spectral peak shift of a producedexemplary pressure-sensor, wherein a pressure spectral sensitivity ofgreater than 500 nm/bar (e.g., about 667 nm/bars) was obtained,according to some embodiments.

FIG. 16 illustrates a measured pressure response of an optical pressuresensor having an ultra-short cavity according to some embodiments usinga miniature spectrometer (Hammatsu C11007MA), and a peak-trackingalgorithm. A sensor-system resolution of better than 10 nm/bar wasachieved, according to some embodiments.

FIG. 17 illustrates a measured spectral peak-shift of the producedexemplary strain-sensor having an ultra-short cavity according to someembodiments. A strain sensitivity of greater than about 0.30 nm/με(e.g., about 0.37 nm/με) was obtained, according to some embodiments.

FIG. 18 illustrates a response of a strain sensing system having anoptical strain sensor having an ultra-short cavity to different strainloads using a miniature spectrometer (Hammatsu C11007MA), and apeak-tracking algorithm, according to some embodiments.

FIG. 19 illustrates a measured spectral-peak shift of the producedexemplary temperature sensor, according to some embodiments.

DETAILED DESCRIPTION

Spectral-integration has proved to be one of the more accurate,reliable, and stable optical-fiber sensor integration methods. Aspectrally-integrated sensor system consists of a sensor that changesits spectral characteristics under the influence of a sensing-parameter.Typical features that are observed regarding these spectralcharacteristics are, for example, positions of one or more local peaks.However, other features such as minimum or minima, slope or otherfeatures, can also be used for the purpose of sensing-parameterdetermination.

Typical, widespread examples of FOS that use this approach are fiberBragg grating (FBGs) sensors. FBG is a periodic structure written withinoptical-fiber that reflects a characteristic optical-wavelength. Thischaracteristic wavelength changes under the influence of a measuredparameter, such as temperature or strain. In order to read, i.e.interrogate, such a FBG sensor, it is necessary to determine thischaracteristic wavelength. Therefore, the integration-system shouldconsist of optical systems that can resolve and analyze back-reflectedoptical spectra. The resolution of the FBG sensor system will depend onthe optical shape of the spectral characteristics (e.g. sharpens of thespectral peak), total system noise, the resolving capability of theanalyzer/integrator, and the spectral sensitivity of the sensor.

Currently, the well-known FOS such as FBGs, Fabry-Perot (FP) sensors,and similar sensors provide relatively low spectral sensitivity. Forexample, a typical FBG will experience a shift of its characteristic'speak wavelength that corresponds to about 10 pm/C and 1 pm/με. Thus,this means that a total shift of the peak wavelength over the entiresensor's operating range is usually within a 1 nm to 5 nm wavelengthband. In order to achieve a useful resolution of such a sensor, aresolving capability of the spectrum analysis system should typically bevery good. For example, the resolution should be within the picometerrange (e.g., range of 1 pm to 10 pm). Thus, conventionally agrating-spectrometer having a complex, long working-distance, and acarefully designed optical design are used to achieve this high level ofresolution. Accordingly, high cost, bulky/large volume design, and largemass are thus frequently associated with such high-resolutionspectrometers. Alternatively, tunable laser sources are used to providerapid laser-emission wavelength scanning over a desired wavelengthrange. Designs having external cavity resonators based upon wavelengthtunable components in the laser's optical feedback are employed in thedesign of such a tunable laser. Again, this design is complex,relatively cost-inefficient and thus incompatible with many cost-drivenapplications. In summary, high-resolution spectroscopic systems areneeded to integrate the great majority of existing spectrally-resolvedFOS's. High-resolution spectroscopic systems are, however, have beenassociated with complex and cost-inefficient designs.

The solution to this unsolved cost-efficiency problem is provided by oneor more embodiments of the invention by providing synergistic designs ofthe optical sensor and signal interrogation apparatus and methods.According to embodiments of the invention, such a solution can beobtained by the application of Fabry-Perot optical sensor with veryshort resonator cavity lengths, and the application of a cost-efficient,low-resolution spectrum analyzer systems, or even color analysissystems. By merging these two technologies, a high-resolution,efficient, and/or environmentally stable (e.g., temperature stable) FOSmeasurement system can be provided, and generally at much lowercomplexity and cost than currently existing solutions.

In particular, one or more of the above described problems are solved byimplementing FOS's that exhibit significant changes in their spectralcharacteristics, so as to allow their spectral integration byapplication of simple spectrum analysis systems, such as miniature,low-resolution, low-cost spectrometers, or even a simple color detectionsystems, for example.

Miniature, low-resolution, low-cost spectrometers based on gratings andsilicon-detector linear arrays have been continuously evolving. Theyhave been successfully commercialized for more than two decades bycompanies such as Ocena-Optics Inc., USA, Hamamatsu, JP, and manyothers. Recent advances in micro-opto-electro-mechanical systems (MOEMS)have accentuated the production of these spectrometers on-the-chip inhighly-integrated, compact, and very cost-efficient ways. Typically,these types of spectrometers operate within a visible and/ornear-infrared range (e.g. between about 400 nm and about 1100 nm), andcan typically reach spectral resolutions of only about 0.5 nm or worse,and more typically around 1 nm. Furthermore, they are usually adaptedfor operating with multimode fibers. The performances of these efficientspectrometers are unfortunately incompatible with mostspectrally-resolved FOS's, such as conventional fiber Bragg gratings(FBG), Fabry-Perot interferometers (FPIs), and similar sensors, asalready explained above.

In order to take advantage of these low-resolution spectrometers or evensimpler spectrum or color-analysis systems, a class of FOS is used thatprovides large and significant changes in their spectral responses whenexposed to measurement parameter change.

According to embodiments of the present invention, such optical sensors(e.g., a Fabry-Perot (FP) optical sensor or Fabry-Perot interferometers(FPIs)), which utilize optical cavities having ultra-short lengths.These and other embodiments of the present invention are furtherdescribed with reference to FIGS. 1-19 below.

According to embodiments of the present invention, a generic FPI with anultra-short cavity length, e.g. an ultra-short FP cavity sensor, isshown in FIG. 1. The optical sensor 100 consists of a lead-in fiber 102that contains a core 104, which functions to guide light, and anultra-short FP cavity 106. The ultra-short FP cavity 106 may be filledwith air, other gases, liquid, or even solid material. The ultra-shortFP cavity 106, as shown, is defined by spaced-apart, semi-reflectivecavity walls 108 and 109. The optical sensor 100 is has a length of theultra-short optical FP cavity 106 that is shorter than 2.5 μm, or evenshorter than 1.5 μm in some embodiments, or even shorter than 1 μm inother embodiments. The cavity length is the distance between the cavitywalls 108 and 109 as measured along a length direction of the lead-infiber 102. Optical sensors 100 with such ultra-short FP cavities 106 canprovide the following distinctive features that can directly affect thesignal interrogation complexity.

First, short FP cavities, like ultra-short FP cavity 106 can have verybroad spectral characteristics that are capable of being resolved oranalyzed by a low-resolution spectrum analysis system. A low-resolutionspectrum analysis system as used herein means resolution of 0.5 nm orworse. For example, the low-resolution spectrum analysis system may havea resolution of 0.5 nm to about 10 nm.

Second, and more importantly, the spectral characteristics ofultra-short FP cavities (e.g., ultra-short FP cavity 106) are verysensitive to cavity-length changes when exposed to the measure parameter(e.g., strain, load, pressure, temperature or the like). For example,shifting of a selected peak the in spectral characteristics of an FPresonator, caused by a change in length of the FP cavity is inverselyproportional to the cavity length of the ultra-short FP cavity 106.Ultra-short FP cavities 106, e.g. cavities that have lengths comparableto the operating wavelength, may thus provide extremely high spectralsensitivities. For example, a nanometer change in the length of a 1 μmlong, ultra-short FP cavity 106 will typical produce greater than ananometer-shift in the position of a spectral peak (e.g., a first orderpeak) of the same cavity when observed within visible wavelength range.

Thirdly, very short FP cavities (e.g., ultra-short FP cavity 106) can beefficiently combined with multimode lead-in fibers, whilst providinggood fringe contrasts and a reasonably clean spectral response. As wasshown in the paper “Exact analysis of low-finesse multimodefiber-extrinsic Fabry-Perot interferometers,” by M. Han and A. Wang,published in Applied Optics Vol. 43, pp. 4659-4666, the fringevisibility, as well as other sensor performance parameters increases asthe cavity length decreases when multimode fiber is used as a lead-infiber in FP sensors. This is mainly due to the fact that light inferenceonly occurs in the same mode whereas different modes could mix when thereflected light is coupled back to the fiber (reduction of cavity lengthreduces this modal mixing).

Compatibility with multimode optical fibers is desired for efficientsystem design, since interrogation of those optical sensors with highspectral sensitivity and broad spectral characteristics requiresbroadband optical sources. Such a broadband optical-spectrum can becost-efficiently obtained, for example by thermal (incandescent) sourcesor light emitting diodes (LEDs) that use indirect light-generationthrough phosphorus luminescence. As such, these light sources generatelimited optical power per unit of surface area and possess very lowspatial coherence. Accordingly, they may only be coupled efficiently tomultimode optical fibers.

Thus, an FPI sensor 100 having an ultra-short FP cavity 106 thusprovides an opportunity for interrogation by low-resolution spectrumanalyzers or even simple color detection systems, while using simplelow-coherence light sources like LEDs or incandescent light sources(e.g. tungsten light bulbs).

Finally, recent advances in all-fiber, all-silica FOS design andmanufacturing, have provided opportunities for effective design andproduction of environmentally very stable, miniature, all-fiber FPIs.These concepts for sensor design and production can be, in many cases,modified in such a way as to provide sensors with ultra-short optical FPcavities 106.

The properties of FP sensors can be described by a few simpleexpressions. The reflectance R_(c) of a short, low finesse, FP cavity,can be expressed as:

$\begin{matrix}{R_{c} = {\frac{2\; {R\left( {1 - {\cos \left( {\frac{4\pi}{\lambda}L} \right)}} \right)}}{1 + R^{2} - {2\; R\; {\cos \left( {\frac{4\pi}{\lambda}L} \right)}}} \approx {2\; {R\left( {1 - {\cos \left( {\frac{4\pi}{\lambda}L} \right)}} \right)}}}} & (1)\end{matrix}$

where R represents the cavity surface's reflectivity, usually defined byFresnel reflection in fiber FPIs, L is the cavity length, and λ thewavelength.

The positions of peaks λ_(m) within the back-reflected optical spectrumcan then be expressed as:

$\begin{matrix}{{\lambda_{m} = {{\frac{4\; L}{\left( {1 + {2\; m}} \right)}\mspace{14mu} m} = 0}},1,{2\mspace{14mu} \ldots}} & (2)\end{matrix}$

where m denotes the peak-order, and L the cavity/resonator's length. Thedistance between the two neighboring spectral peaks, can be furtherexpressed as:

$\begin{matrix}{{\Delta\lambda}_{m:{m + 1}} = \frac{2\; L}{\left( {m + \frac{1}{2}} \right)\left( {m + \frac{3}{2}} \right)}} & (3)\end{matrix}$

The spectral peak's positional shift due to the optical cavity lengthchange (e.g. spectral sensitivity,) can be described as:

$\begin{matrix}{{d\; \lambda_{m}} = {{\frac{4}{\left( {1 + {2\; m}} \right)}{dL}\mspace{14mu} {or}\mspace{14mu} d\; \lambda_{m}} = {\frac{\lambda_{m}}{L}{dL}}}} & (4)\end{matrix}$

and a full width at half-maximum amplitude (FWHM) for a peak of theorder m, can be expressed as:

$\begin{matrix}{{FWHM}_{\lambda_{m}} = \frac{L}{\left( {m + \frac{1}{4}} \right)\left( {m + \frac{3}{4}} \right)}} & (5)\end{matrix}$

The spectral sensitivity is thus inversely-proportional to the peakorder m. For example, for m=1, a change of cavity length dL for 100 nmwill produce a spectral peak shift dλ of about 133 nm, a typical valuethat can be easily resolved by existing, cost-efficient, miniaturespectrometers. However, in order to take advantage of these highsensitivity low-order spectral peaks, they need to appear within avisible or near-inferred range, which can only be achieved by a drasticreduction in the initial FP cavity length, preferably down to or evenbelow the operating optical wavelength range, as indicated by expression(2) above.

Ultra-short cavities (e.g., ultra-short FP cavity 106) produce largespectral peak shifts. In extreme case, for m=0 (in this case the peakwavelength corresponds to 4L), we obtain dλ=4dL. Since a typicallow-cost spectrum analyzer provides only roughly nanometer resolution, ameasurement system using optical sensors having such an ultra-short FPcavity 106 should provide length measurement resolutions within therange of a few hundred picometers.

Short cavities also produce broad free-spectral ranges that allow for anunambiguous determination of cavity length, even in the cases ofrelative cavity length changes. FIG. 2 shows the modeled spectrum of a0.75 μm-long low-fines FP cavity. The peak wavelength position of thefirst-order peak (m=1) corresponds to 1 μm. The neighboring peaks'wavelengths are located at 0.6 μm and 3 μm. Shortening of the cavitylength by 100 nm, which corresponds to 13% of the initial cavity length,causes a peak of the order m=1 to move by 133 nm, to a new position thatis still sufficiently far away from the original neighboring peaks'positions.

A demonstration of cavity length influence on the spectralcharacteristics of various short-cavity FP sensors, like optical sensor100 are shown in FIG. 3 to FIG. 8. These figures show modeled spectralcharacteristics of cavities with various lengths within a wavelengthrange between about 400 nm and 1100 nm (e.g. typical working range ofminiature, cost-efficient low-resolution, spectrometers). FIGS. 3through 8 also demonstrate changes in the various cavities spectra whenthe absolute cavity lengths are increased by 100 nm. Ultra-shortcavities, e.g. cavities with lengths below 2 μm, provide thelowest-order peaks of m<5 within a 400 nm to 1100 nm wavelength range.These figures also exhibit relatively large spectral peak shifts (e.g.,that are greater than 0.4 nm per each nanometer of cavity lengthchange). When the length of the cavity is reduced to below 1 μm, thenumber of peaks within the 400 nm to 1100 nm spectral range is furtherreduced and the lowest-order peaks, such as a second-order peak (m=2)with spectral sensitivity of about 0.85 nm/nm, appears within the 400 nmto 1100 nm wavelength range. Further reduction in cavity length canprovide a presence of the two lowest (first and zero-order) spectralpeaks (m=0 and m=1) within the 400 nm-1100 nm spectral range. Thefirst-order peak's sensitivity is about 133 nm/nm. The zero-order peak(m=0) has the highest attainable spectral sensitivity, that correspondsto about 4 nm/nm. In order to take advantage of the first-order peak(m=1) within a 400 nm to 1100 nm spectral range, the cavity length maybe reduced to or below about 0.75 μm. In order to use the zero-orderpeak (m=0) within a 400 nm to 1100 nm spectral range, a cavity's lengthmay be reduced to about 0.25 μm.

FIG. 3 also demonstrates that FP sensors 100 having a cavity lengthchange from 0.35 μm to 0.25 μm may cause a complete change or reversalof the entire spectral characteristics. The 0.35 μm-long cavity stronglyreflects short wavelengths, while producing a reduction in the cavitylength by only 100 nm, suppressing the reflection of the short portionof the spectrum, and promoting longer wavelengths reflection. In one ormore embodiments, such changes can be directly detected/measured bysimple color detection systems, such as RBG sensor or similarcolor-detection sensors, which are far less complex and lower-cost thanspectrum analyzers. Systems including color detection apparatus and FPoptical sensors having a cavity length change from 0.35 μm to 0.25 μmmight not only be limited to zero-order peak observation, but also tothe observation of spectra that might contain a few lower-order peaks,like m=1 or perhaps even m=2.

Table 1 below summarizes a few more typical properties of ultra-short FPcavity length spectral characteristics.

TABLE 1 Positions of spectral peaks and FWHM at m = 1 for a fewexemplary FPI sensor with ultra-short FP cavities L (μm) λ_(m=0) (μm)λ_(m=1) (μm) λ_(m=2) (μm) FWHM_(λm=1) (μm) 0.25 1 0.33 0.2 0.114 0.5 20.67 0.4 0.228 0.75 3 1.00 0.6 0.340 1.0 4 1.33 0.8 0.457

In summary, optical sensors having ultra-short FP cavities (e.g.,optical sensor 100) may be used to provide for high-resolutionmeasurements of optical path-length changes, while using cost-effective,low-resolution spectrum analysis systems operating within a visible andnear-infrared range (e.g., 400 nm to 1100 nm). Furthermore, theapplication of ultra-short cavity FP interferometers allows for theusage of multimode lead-in fibers, which may allow for reasonablyeffective coupling of light from low-cost, ultra-broadband sources(e.g., LED and other low-cost light sources). Furthermore, in someembodiments, spectrum analyzers can be replaced by even simpler systems,such as color analysis devices that can simultaneously detectlight-intensity at more than one wavelength. For example Red, Green, andBlue (RGB) detectors may be used that can simultaneously detectlight-intensity at three different wavelengths. Such RGB detectors arewidely available in the form of a single chip, and can be utilized toperform such color analysis. Other types of color sensors or detectorsmay be used. In this case, ratios between individual color channels orother signal processing algorithms can be used to determine the cavitylength change. Lower-order FP cavities (e.g., m=0 and m=1) arebest-suited for such applications as they generate optical spectra,which extend across the entire or most of the wavelength range coveredby the RBG detector. RBG detectors that provide non-filtered lightdetection channels or additional color channels can also be used (e.g.,RGBY sensors).

With respect to the above analysis, ultra-short cavity Fabry-Perotoptical fiber sensors, as further defined above, are sensors that haveinitial cavity lengths between surfaces of the cavity walls 108 and 109that are shorter than 2.5 μm, shorter than 1.5 μm in some embodiments,and shorter than 1 μm in other embodiments. These ultra-short cavitylengths allow for spectral interrogation of the cavity length changes byobservation of the cavity spectral characteristics within the wavelengthrange in which cavities' spectral peaks of the order m=0 to m<5 occurduring the operation of the sensor.

According to embodiments of the present invention, the optical system900, shown in FIG. 9 consists of an ultra-short cavity Fabry-Perotoptical sensor such as the sensor 100 described above, which changes incavity length under the influence of a measured parameter (e.g., understrain, load, pressure, temperature or the like), and an opticalinterrogation system 901. The optical interrogation system 901 includesa broad-band light source 912, a spectrally-sensitive detection system914 that has low resolution (e.g., resolution of 0.5 nm or worse) andcan resolve or analyze at least part of the optical spectrum emitted bya broad-band light source 912, an optical separator 916 (e.g., anoptical coupler or beam splitter) that directs optical radiation fromthe broad-band light source 912 to the ultra-short cavity Fabry-Perotoptical sensor 100 and back to the spectrally-sensitive detection system914, a multimode optical fiber 918 that interconnects with at least anultra-short cavity Fabry-Perot optical sensor 100 and optical separator916, and a signal processing system 920 (e.g., electronics and/or microcomputer based signal processing system) that can analyze and processsignals generated by spectrally-sensitive detection system 914. Opticalfiber 922 may also interconnect the broad-band light source 912 and theoptical separator 916 and spectrally-sensitive detection system 914. Thespectrally-sensitive detection system 914 can be any type of spectrumanalyzer, such as grating-based spectrum analyzer, MOEMS or MEMS-basedspectrum analyzer, or any other device that can analyze optical spectra.The spectral resolution of such a spectrum analyzer can be relativelylow, such as within the range of 0.5 nm to 10 nm.

The ability to use a spectrally-sensitive detection system 914 havingsuch low spectral resolution is due to the high spectral sensitivity ofultra-short Fabry-Perot cavity optical sensors 100 that providesignificant changes in spectral characteristics, even at small changesin cavity lengths. Furthermore, spectrally-sensitive detection system914 can be comprised of two or more spectrally-sensitive detectors withtwo or more shifted peak-sensitivity wavelengths, such as RGB sensor, animproved RGB sensor, or other multi-color optical sensor or multi-colordetector. The broad-band light source 912 can be an optical source, suchas a light bulb, a xenon-filled light bulb, or a broad-bandlight-emitting diode (LED). Other broad band light sources may be used.Broad-band source, as used herein means a light source that emits anoptical spectrum having a Full Width at Half Maximum amplitude (FWHM) ofgreater than about 250 nm, greater than about 500 nm, or even more, andthat emits wavelengths within range of between about 350 nm and about1100 nm.

The ultra-short Fabry-Perot cavity optical sensor 100 is an opticalsensor having an optical cavity length that is shorter than 2.5 μm,shorter than 1.5 μm, or even shorter than 1 μm. The initial(un-stressed) cavity length of the ultra-short cavity Fabry-Perotoptical sensor 100 should be such that at least one of the zero-order(m=0), first-order (m=1), second-order (m=2), third-order (m=3),fourth-order (m=4) or fifth-order (m=5) peaks coincides with theoperating wavelength range of spectrally-sensitive detection system 914and broad-band light source 912. Such optical sensors 100 may exhibitrelatively high spectral sensitivity, e.g. have a spectral peak shift asa consequence of cavity length change that is 400 μm per nanometer (nm)of cavity length change or more. In some embodiments, when observingfirst-order cavity peak (m=1) shifts, the spectral peak sensitivity maybe over 1 nm per 1 nm of the cavity length change. Furthermore, inanother preferred case of zero-order peak (m=0) tracking/observation,the peak position can change by approximately 4 nm per each nm of actualcavity length change.

Optical sensors 100 having ultra-short cavity lengths at or aroundoptical wavelengths in visible and near-infrared ranges should haveproperties of dimensional and thermal stability within nanometer range.Secondly, the ultra-short cavity initial cavity length should providenanometer resolution, in order to obtain the desired initial spectralresponse, that is 1 nm or more response for each nm of length change.Finally, in spite of ultra-short cavity length, the sensor should remainsensitivity to the measured parameter (e.g., strain, temperature,pressure, or the like).

While there are many methods for producing fiber-optic Fabry-Pertsensors, including, MEMS and MOEMS, laser and other micromachiningtechnologies, methods that allow the production of all-silica, all-fibersensors may be best-suited for the realization of ultra-short cavitysensors as described herein. Such sensor examples are described in U.S.Pat. No. 7,684,657, in U.S. patent application Ser. No. 13/046,648entitled “Optical Fiber Sensors Having Long Active Lengths, Systems, AndMethods,” filed on Mar. 11, 2012,” and in “All-fiber, long-active-lengthFabry-Perot strain sensor. Opt. express”, 2011, vol. 19, no. 16, pp.15641-15651 by Pevec and Donlagic, the disclosures of all of which arehereby incorporated by reference in their entirety herein. These typesof optical sensors may be manufactured to mostly meet all the aboverequirements. All-silica optical sensor design provides good thermal andenvironmental stability. Furthermore, since these optical sensors may beassembled by fusion-splicing, the latter eliminates using potentiallythermally and chemically unstable bonding materials.

The production, testing, and interrogation of several exemplaryultra-short cavity FP sensors will now be described. The first opticalsensor 1100 is a pressure sensor and is derived from the sensorsdescribed in U.S. Pat. No. 7,684,657. The optical sensor 1100 is shownin FIG. 11 and consists of lead-in fiber 1102 and a diaphragm 1109positioned in the front of the core 1104 of the lead-in fiber 1102. Theend-surface of the lead-in fiber 1102 and the diaphragm 1109 form anultra-short FP cavity 1106. The cavity length of the ultra-short FPcavity 1106 is shorter than 2.5 μm, shorter than 1.5 μm in someembodiments, and shorter than 1 μm in other embodiments. The diaphragm1109 may be attached to the lead-in fiber 1102 via a spacer or otherlike structure. In some embodiments, the spacer and diaphragm 1109 maybe formed as a single-piece optical element and made of silica glass.The optical sensor 1100 may be produced by micromachining process thatis based on selective etching using specially produced sensor-formingfiber. This fiber may have a large doped (step index) core that etchesat a higher rate than pure silica when exposed to hydrofluoric acid(HF), which forms a cavity at the tip of the fiber after wet-etching.The etching time and dopant concentration are used to control theinitial cavity depth. This etched fiber is then fusion-spliced to thelead-in fiber 1102 (e.g., a telecommunication optical fiber having a 50um outer glass diameter) to form the FP cavity 1106. The etched fiber iscleaved near the splice and polished to obtain a pressure-sensitivediaphragm 1109. Furthermore, the lead-in fiber 1102 containing thediaphragm and cavity may be etched in order to increase a sensitivity ofthe sensor 1100 to at least 500 nm/bar. This process is described indetail in U.S. Pat. No. 7,684,657 and may be used to yield cavitylengths of the order described herein (e.g., less than 2.5 μm).

To further reduce the cavity length to a within a sub-micrometer range,the etching time of the sensor-forming fiber may be shortened to obtainan initial cavity depth of less than about 2.5 μm. Furthermore, anactive, feedback-assisted, splicing process and apparatus may beapplied. The apparatus 1000, as shown in FIG. 10, includes acomputer-controlled filament fusion splicer 1025 (e.g., a Vytran FSS2000), a low-resolution spectrum analyzer 1014 (E.G., A HamamatsuC11007MA), an optical separator 1016 (e.g., a 3-dB coupler), and atungsten-filament light bulb as an optical light source 1012. The etchedsensor-forming fiber 1005 is fusion-spliced to the lead-in fiber 1102while carrying out on-line observation of the back-reflected spectra.

The fusion process consists of the usual fusion splicing steps with theaddition of a final cavity length-tuning step. During this tuning stepthe fusion temperature is reduced, whilst commanding the fusionsplicer's longitudinal motors to gradually compress the splice/cavity.The fusion process is terminated when a desired sensor spectra/lengthhas been obtained. This additional step may be used to directly producesubmicron cavity lengths, by a combination of initial shallow-etching ofthe sensor-forming fiber 1005 and length control between the cavitysurfaces during such active fusion-splicing. The active fusion processenables precision-tuning of the initial cavity length and, consequentlythe spectral peak's position. FIG. 12 shows optical spectra of thecavity (e.g., FP cavity 1106) before and after splicing. The spectrumbefore splicing as designated by line 1230 is obtained by compressingthe lead-in fiber 1102 and etched sensor-forming fiber 1005 beforeinitiating the fusion process to a first length. The spectrum indicatesa first cavity length of about a 1.9 μm. During splicing, the FP cavity1106 is compressed until the first-order peak (m=1) appears within thespectral range of the spectrum analyzer. The spectrum after splicing asdesignated by line 1235. The spectrum indicates a second cavity lengthafter splicing of about a 0.46 μm. The remaining part of the sensorproduction process may be the same as described in U.S. Pat. No.7,684,657, including the final etching to achieve sensitivity of about500 nm/bar or more.

A second ultra-short cavity FPI sensor as shown in FIG. 13A is a strainsensor. In order to obtain relatively high-strain sensitivity, an activelength and the cavity length need to be separated; otherwise the cavitylength also determines the active length. Such a design can be achievedeither by fiber-in-capillary or fiber-in-ferule designs, as described inU.S. Pat. No. 5,301,001, for example, or by proper micromachining of theoptical fiber. The micromachining can be a particularly suitable methodfor the design and production of an ultra-short cavity sensor as, forexample, described in “Optical Fiber Sensors Having Long Active Lengths,Systems, And Methods,” filed on Mar. 11, 2012, and in “All-fiber,long-active-length Fabry-Perot strain sensor. Opt. express”, 2011, vol.19, no. 16, pp. 15641-15651 by Pevec and Donlagic.

A side view of such a sensor 1300 is shown in FIG. 13A. This sensorconsists of a FP cavity 1306, defined by an end surface 1307 of a core1304 of a lead-in fiber 1302, a micro-machined retracted sensor-formingfiber-end surface 1309, and a deep gutter 1313 that surrounds theretracted fiber-end surface 1309. This optical sensor 1300 ismicro-machined out of a specially designed sensor-forming fiber thatconsists of a central part, which is lightly doped with dopant that hasa limited effect on the etching rate (for example by TiO₂). This centralpart may be further surrounded by a phosphorus pentoxide-doped ring,which strongly increases an etching rate of the silica (typically byover 40 times over pure silica etch rate) when exposed to HF.Wet-etching of such fiber in an acid (e.g., HF) may result in astructure that consists of the slightly retracted fiber-end 1309, whichis further surrounded by the deep gutter 1313. The fusion of such anetched fiber with lead-in fiber 1302 yields an optical strain sensor1300 that has an active length determined by gutter depth, whilst thecavity length can almost be arbitrary and is settable by dopant-levelcontrol within the central portion of the sensor-forming fiber.

The ultra-short FP cavity strain sensor 1300 that utilizes anultra-short cavity length (e.g., less than about 2.5 μm) may be producedby same method as described above. However, the sensor-forming fiber maybe optimized for ultra-short cavity production. In particular, theoptimized sensor-forming fiber used has identical parameters to thosereported in the above-cited Donlagic and Pevec reference, except that itmay reduce the TiO2 concentration in the central region to about 1.5 mol% or less, in order to reduce any retraction of the central part of thefiber during etching. Furthermore, the feedback assisted, fine-tuningstep during the splicing process may be added in the same way as whenproducing pressure-sensors, as described previously. FIG. 13Billustrates and actual micrograph of such an optical sensor 1300.

A simple temperature-sensor 1400, as shown in FIG. 14, may be providedby application of the above described strain sensor 1300 and fixing(e.g., gluing) the strain sensor 1300 into a ferula 1436 (e.g., azirconia ferula). The ferula 1436 may have a 1.25 mm diameter, forexample. The fibers coupled to the sensor 1300 may be fixed on each sideof the ferula with an adhesive 1438 (e.g., epoxy). The ferula 1436 maybe about 10.5 mm in length. Zirconia has a relatively large coefficientof thermal expansion (CTE), e.g. 10̂−5 K⁻¹, which is significantly higherthan the CTE of silica (5×10̂−7 K⁻¹). The temperature change of theferula 1436 thus induces strain on the strain sensor 1300 fixed in theferula 1436. The length of the cavity of the sensor 1300 may be lessthan 2.5 μm.

Several experimentally produced examples of sensors were tested forpressure, strain, and temperature responses. All the initial sensorcavity lengths were set to allow observation of the first-order spectralpeaks (e.g. m=1) within the visible band of the optical spectrum. Anygradual increases in the pressure, strain, and temperature causes thesespectral peaks/fringes to shift, as indicated in FIGS. 15-20.

FIG. 15 illustrates a first-order (m=1) spectral peak shift (nm) versusapplied pressure (bar) of one produced exemplary pressure sensor havingan ultra-short cavity length. The experimentally measured pressurespectral sensitivity was greater than about 500 nm/bar (e.g., about 667nm/bar).

FIG. 16 demonstrates a measured response of a pressure sensor having anultra-short cavity as described above when the pressure was changed(increased and then decreased) to 10, 20, 40, 60, and 80 mbar. Thesampling rate was 1 Hz, the used spectrum analyzer was a HammatsuC11007MA, the lead-in fiber was 50 μm telecom multimode fiber, and atungsten bulb was used as a broad-band light source. FIG. 16 indicatesthat a resolution significantly better than 10 mbar can be obtained.

FIG. 17 shows a first-order (m=1) spectral-peak shift (nm) versusapplied strain (μm/m) of a produced exemplary strain sensor having anultra-short cavity length as described above. The spectral sensitivityto the strain was approximately 0.37 nm/με (about 0.37 nm/(μm/m)).

FIG. 18 demonstrates the measured response of the strain sensor whencyclically exposed to various loads causing strain variations. Thesampling rate was 1 Hz, the used spectrum analyzer was a HammatsuC11007MA, the lead-in fiber was 50 um telecom multimode fiber, and atungsten bulb was used as the broad-band light source. FIG. 18 indicatesthat a resolution of better than 3με can be achieved (about 0.37nm/(μm/m)).

FIG. 19 shows the first-order (m=1) spectral-peak wavelength shift (nm)versus applied temperature (° C.) of a produced exemplary temperaturesensor having an ultra-short length cavity as described above. Thespectral-sensitivity to the strain was greater than about 8 nm/° C., oreven greater than about 10 nm/° C., and about 14 nm/° C. in the depictedembodiment.

The results shown above in FIGS. 15, 17 and 19 and other plots wereobtained by simple tracking of spectral peaks. In such simple casesunambiguous cavity length measurement ranges may be limited by theresonators' free spectral-ranges. This is particularly valid for long FPcavities, when a change in resonator length for one half of thewavelength reproduces nearly identical spectral characteristics thatcannot be easily distinguished amongst each other. It should beunderstood that in ultra-short cavity FP sensors, a free spectral-rangedoes not impose such a limitation. In ultra-short cavity sensor changein cavity length for halves of the wavelengths induces significantchanges within the spectral footprint that can be easily recognized bysimple algorithms. For example, a cavity with a length of 0.5 μm hasvery different spectral characteristics from a 1 μm long cavity, asindicated for example in FIGS. 4 and 6. Proper signal-processingalgorithms can be easily applied, and can provide continuousmeasurements of cavity length changes in excess of one or multiplehalf-wavelengths.

The foregoing description discloses only exemplary embodiments of theinvention. Modifications of the above-disclosed apparatus, systems, andmethods which fall within the scope of the invention will be readilyapparent to those of ordinary skill in the art. Accordingly, while thepresent invention has been disclosed in connection with exemplaryembodiments thereof, it should be understood that other embodiments mayfall within the scope of the invention, as defined by the followingclaims.

The invention claimed is:
 1. An optical system, comprising: an opticalsensor having an ultra-short Fabry-Perot cavity, wherein the ultra-shortFabry-Perot cavity comprises a cavity length of shorter than 2.5 μm; andan optical interrogation system coupled to the optical sensor andoperational to send light signals to the optical sensor and receivelight signals from the optical sensor within a wavelength range of atleast between 400 nm to 1100 nm, which includes visible andnear-infrared range, wherein the optical interrogation system comprisesa spectrally-sensitive detection system having a color detector adaptedto receive the light signals and simultaneously detect light-intensityat more than one wavelength.
 2. The optical system of claim 1, whereinthe color detector comprises a multi-color detector.
 3. The opticalsystem of claim 1, wherein the color detector comprises a multi-coloroptical sensor.
 4. The optical system of claim 1, wherein the colordetector comprises an RGB sensor.
 5. The optical system of claim 1,wherein the color detector comprises an RGBY sensor.
 6. The opticalsystem of claim 1, wherein the color detector is configured to detectlight intensity at three different wavelengths.
 7. The optical system ofclaim 1, wherein the optical interrogation system is configured todetermine ratios between individual color channels.
 8. The opticalsystem of claim 1, wherein the color detector and cavity length areconfigured to detect a zero-order peak within the wavelength range of atleast between 400 nm to 1100 nm.
 9. The optical system of claim 1,wherein the color detector and cavity length are configured to detect afirst-order peak within the wavelength range of at least between 400 nmto 1100 nm.
 10. The optical system of claim 1, wherein the colordetector and cavity length are configured to detect a second-order peakwithin the wavelength range of at least between 400 nm to 1100 nm. 11.The optical system of claim 1, wherein the color detector and cavitylength are configured to detect lower-order peaks within the wavelengthrange of at least between 400 nm to 1100 nm.
 12. The optical system ofclaim 1, comprising: multimode lead-in fibers coupled to the opticalsensor.
 13. The optical system of claim 1, comprising: a broad-bandlight source operational to send the light signals.
 14. The opticalsystem of claim 1, wherein the cavity length is shorter than 1.5 μm. 15.The optical system of claim 1, wherein the cavity length is shorter than1.0 μm.
 16. The optical system of claim 1, wherein an initial cavitylength of the optical sensor is of a length such that at least one of azero-order (m=0), a first-order (m=1), a second-order (m=2), athird-order (m=3), a fourth-order (m=4), or a fifth-order (m=5) peakcoincides with an operating wavelength range of between 400 nm to 1100nm.
 17. The optical system of claim 1, wherein when observing afirst-order cavity peak (m=1) shift, the optical sensor has a spectralpeak sensitivity of over 1 nm per 1 nm of cavity length change in nm.18. The optical system of claim 1, wherein the optical sensor has aspectral peak shift, as a consequence of a cavity length change, of 400μm per nanometer or more, and the optical interrogation system has aspectral resolution of between 0.5 nm and 10 nm.
 19. An opticalassembly, comprising: an optical sensor having an ultra-shortFabry-Perot cavity having an initial cavity length shorter than 2.5 μm;and an optical interrogation system coupled to the optical sensor andoperational to send light signals to the optical sensor and receivelight signals from the optical sensor within a wavelength range of atleast between 400 nm to 1100 nm, which includes visible andnear-infrared range, the optical interrogation system comprising aspectrally-sensitive detection system having a multi-color detectoradapted to receive the light signals and simultaneously detectlight-intensity at multiple wavelengths.
 20. A method of interrogatingan optical sensor, comprising: providing an optical sensor having anultra-short Fabry-Perot cavity, wherein the ultra-short Fabry-Perotcavity comprises a cavity length of shorter than 2.5 μm; andinterrogating the optical sensor by sending light signals to andreceiving light signals from the optical sensor within a wavelengthrange of at least between 400 nm to 1100 nm, which includes visible andnear-infrared range, wherein the interrogating is carried out by anoptical interrogation system comprising a spectrally-sensitive detectionsystem having a color detector adapted to receive the light signals andsimultaneously detect light-intensity at more than one wavelength.